A thermally insulating wall construction (1, 100, 199, 200 or 300), a masonry panel (5A, 513, 205A, 20513, 305A, 305B) and a method of fabricating masonry panels are disclosed. The panels are fabricated by pouring lightweight concrete into a mould (201) which includes a former (227) preferably formed from egg carton cardboard, or similar. The former (227) is retained in situ. Film or foil (28, 228) in one or several layers is positioned in a cavity (12) formed between opposed panels to thereby create a plurality of thermal barriers in stagnant air. High thermal ratings for wall constructions are thereby able to be achieved.